Page 314 - Practical Ship Design

272 Chapter 9

consumption. This disadvantage - already quite significant at full power operation
power - becomes much worse for most gas turbine types when they are run at part
load and if operation at part load power is a frequent requirement this has generally
eliminated them from consideration.
A few merchant ships were built with gas turbine propulsion in the late
sixties/early seventies and were successful technically, but became quite unecon-
omic when fuel prices rose dramatically in 1973 and the ships concerned have
since either gone out of service or been re-engined with diesels.
The situation with warships is quite different with the high powedweight ratio
of the gas turbine making this very attractive on a ship where any additional weight
increases the power required so much that it becomes easy to get into a vicious
spiral. The poor specific fuel consumption of a gas turbine does not matter as much
on a warship as it does on a merchant ship because warships tend to use full power
infrequently and for relatively short bursts. It has, however, become quite usual
practice for warships to have two separate sets of machinery - one for maximum
speed and one for the cruise regime. These two sets of machinery can consist either
of two gas turbines, a high power set for the high-speed regime and a lower power
set for the cruise regime, or more frequently a combination of high-power light-
weight gas turbines for maximum speed with high or medium-speed diesels with
good specific fuel consumption for the cruise regime. The latter combination
clearly making very good sense.
Both of these combinations can be arranged in two ways with the two machinery
fits as alternatives (COGOG) or (CODOG) or with them so linked that both are
used to develop the maximum power (COGAG) or (CODAG).
Although the latter combination offers more power there are complications in
the gearing required and at present the former is the generally preferred option. On
frigates there are usually either two gas turbines or a gas turbine and a diesel geared
to each of the twin shafts; on corvettes one gas turbine and one diesel may be fitted
with a central gearbox dividing the power between the two shafts.
Gas turbines are generally arranged as modules suitable for repair-by-
replacement with the machinery casings sized to suit.
As most gas turbines for marine use have been developed from aircraft engines,
the number of models is quite limited and come principally from Rolls Royce and
General Electric. The powers of those currently being fitted are given in Table 9.1.
A return to the use of gas turbines on merchant ships may not be far away as it
seems likely that developments presently in hand will reduce the specific
consumption of a gas turbine to much the same as that of a diesel engine and go a
long way towards eliminating the reduction in efficiency at part power.
In addition, as discussed later under electrical propulsion, the low weight and
particularly the low space requirements of these prime movers is starting to look
attractive for cruise ships.